Engineering Commons^™

Nanostructured Chitosan Membranes For Filtration, Keyur Desai

Doctoral Dissertations

Chitosan is a non-toxic and biodegradable biopolymer derived from naturally occurring chitin. It has excellent metal binding and anti-microbial properties which could be beneficial in air and water filtration applications. Nanofibers have distinctly high surface area to volume ratio.

Electrospinning is a process by which nano-sized polymer fibers can be produced using an electrostatically driven jet of polymer solution. The fibers are collected as a non-woven mat and offer a high surface area to volume ratio.

Electrospinning of pure chitosan is hindered by its limited solubility in aqueous acids and high molecular weight with high degree of inter and intra …

A Study Of The Indentation Size Effect In Copper, Jeremy Harper Strader

Doctoral Dissertations

A simple model for an expanding array of screw dislocations is developed to evaluate the size of the containment zone of the geometrically necessary dislocations as it relates to the indentation size effect. The model as developed predicts the same functional form as the Nix-Gao model, H²=H₀²(1+h^*/h), but a new relation for h^* applies that depends only on the Burgers vector and the Taylor constant, where as the Nix-Gao model predicts a strong dependence of the macroscopic hardness and the tip angle. The continuous stiffness method, CSM, is critically examined using experimental …

Mechanical Behavior Of A Ni-Based Crystalline And A Zr-Based Amorphous Materials Subjected To Surface Severe Plastic Deformation, Jiawan Tian

Doctoral Dissertations

A surface-treatment process, surface-severe-plastic deformation (S²PD), is developed and applied on both crystalline and amorphous materials to introduce the plastic deformation in the near-surface layer. A S²PD-processed crystalline component is expected to have enhanced fatigue properties because the refined grains in the near-surface layer and the coarse grains in the interior have good resistance to the crack initiation and propagation, respectively.

The near-surface structures of the processed specimens were characterized by means of the optical microscopy, scanning-electron microscopy (SEM), X-ray diffraction (XRD), and transmission-electron microscopy (TEM). Mechanical properties, such as the microhardness, yield strength, and four-point-bend …

Localized Corrosion Behavior Of Zr-Based Bulk Metallic Glasses In Neutral Nacl Electrolytes, Brandice Annette Green

Doctoral Dissertations

The electrochemical behaviors the Zr₅₀Cu₄₀Al₁₀, Zr₅₀Cu₃₃Al₁₀Pd₇, and Zr_52.5Cu_17.9Al₁₀Ni_14.6Ti_5.0 BMGs were investigated in NaCl solutions. Both Zr₅₀Cu₄₀Al₁₀ and Zr₅₀Cu₃₃Al₁₀Pd₇ exhibited poor corrosion behaviors. Zr₅₀Cu₄₀Al₁₀ was vulnerable to localized corrosion at the open-circuit conditions (OCC). Zr₅₀Cu₃₃Al₁₀Pd₇ experienced localized corrosion at OCC or at slightly higher potentials. Anodic-polarization of both alloys demonstrated that small increases in …

Magnetic Field Effects On Photocurrent And Photovoltaic Processes In Organic Solar Cells, Zhihua Xu

Doctoral Dissertations

Two mechanisms were investigated to understand the magnetic field effects on photocurrent (MFP), a phenomenon whereby an external magnetic field (MF) can increase or decrease photocurrent in organic solar cells (OSCs). The first one relates to MF-modulated intersystem crossing (ISC) between singlet and triplet polaron pairs and charge transfer (CT) complexes, accounting for the positive MFP in low and high magnetic field, respectively. The second mechanism originates from the interaction between triplet excited states and charge carriers, namely triplet-charge reaction (TCR). External magnetic field was found to reduce the rate constant of TCR, leading to negative MFP.

MFP measurements were …

Deformation And Fatigue Behavior Of Bulk-Metallic Glasses (Bmgs), Dongchun Qiao

Doctoral Dissertations

The objectives of the thesis research are 1) to study the strain rate effect on the deformation behavior of bulk-metallic glasses (BMGs), 2) to investigate the loading mode effect on the fatigue behavior of BMGs.

The main results are obtained as follows. At room temperature, during the inhomogeneous deformation, BMGs show the strain softening caused by the increase of the free volume in the shear bands. However, BMGs have a high-energy state and trend to crystallize, especially in the shear bands with the maximum shear stress and the increasing temperature. The instability of BMGs makes it possible for strain hardening. …

Elastic Properties Of Bulk-Metallic Glasses Studied By Resonant Ultrasound Spectroscopy, Zhiying Zhang

Doctoral Dissertations

The elastic properties of a solid are of considerable interest to both science and technology. Not only do they contain fundamental information about the nature of the inter-atomic bonding in the material, but they also determine the mechanical behavior of solids. In the past few years, considerable effort has been devoted to the study of elastic properties of bulk metallic glasses (BMGs), a relatively new class of metallic materials that display a unique combination of mechanical and physical properties. Our research has focused on Zr-based, Cu-based and Ca-based metallic glasses. Zr-based BMGs are known to have superior glass forming ability …

An Experimental Investigation Of The Elastic And Viscoelastic Properties Of Small, Erik Gregory Herbert

Doctoral Dissertations

The purpose of this work is to further develop experimental methodologies using flat punch nanoindentation to measure the constitutive behavior of viscoelastic solids in the frequency and time domain. The reference material used in this investigation is highly plasticized polyvinylchloride (PVC) with a glass transition temperature of -17 degrees Celsius. The nanoindentation experiments were conducted using 103 and 983 micron diameter flat punches. For comparative purposes, the storage and loss modulus obtained by dynamic mechanical analysis are also presented. Over the frequency range of 0.01 to 50 Hz, the storage and loss modulus measured using nanoindentation and uniaxial compression are …

Microstructural Control And Alloy Design Of The Ti-Al-Nb-W-B Alloys, Lan Huang

Doctoral Dissertations

The TiAl alloys have been considered as promising candidates for structural-materials applications at around 800⁰C. The major concern for the structural use of the TiAl alloys is their low ductility and poor fracture resistance at ambient temperature. Refining the grain size of the TiAl alloys can be an effective way to improve the mechanical properties of the alloys. In this work, new TiAl alloys, containing tungsten (W) and boron (B), have been developed. Using the scanning-electron microscopy (SEM), electron-microprobe, and transmission-electron microscopy (TEM), the effects of W and B on the microstructural evolution of TiAl alloys, including the …

Strain-Induced Phase Transformation And Anisotropic Lattice-Strain Development In A Cobalt-Based Superalloy, Michael Lee Benson

Doctoral Dissertations

This dissertation addresses two issues concerning the deformation behavior of a cobalt-based superalloy: 1) the strain-induced face-centered-cubic to hexagonal-closepacked phase transformation and 2) the anisotropic lattice-strain development. Cobaltbased alloys are known to exhibit a metastable face-centered-cubic crystal structure at room temperature. This structure can transform to the thermodynamically-stable hexagonal-close-packed phase upon application of a load. While this phenomenon has been studied and documented before, this work offers a more in-depth study on this issue, as it addresses the phase transformation under several loading modes: monotonic tension, monotonic compression, stress-controlled high-cycle fatigue, and strain-controlled lowcycle fatigue. The experimental technique utilized in …

Processing, Structure, And Properties Of Amorphous Aluminum Alloys, Timothy Wayne Wilson

Doctoral Dissertations

Although research has been conducted on amorphous aluminum-based alloys, most of the research has focused on melt-spun ribbons. There has been significantly less research on mechanically alloyed amorphous powder even though mechanically alloyed powder seems to have more potential for the production of bulk amorphous aluminumbased alloys. In addition, there has not been adequate research conducted on the local atomic structure of amorphous aluminum alloys, and a greater understanding of the relationship between processing, structure, and properties is necessary.

In the following thesis, multiple investigations have been performed to understand the structure, processing, and properties of aluminum-based amorphous alloys. These …

Fundamental Deformation Micromechanics In A Zircaloy-4 Alloy And The Hydrogen Effects On Its Microstructure, Internal Stresses, And Fatigue Behavior, Elena Garlea

Doctoral Dissertations

Zircaloy-4 alloys, polycrystalline zirconium alloys, are extensively used in the nuclear industry. During the service in the reactor, these alloys absorb hydrogen, leading to formation of zirconium hydrides, which may be enhanced by the stress field around a crack tip. In order to investigate these phenomena in a Zircaloy-4 alloy, the effect of internal stresses on the hydride precipitation and the subsequent influence on the fatigue behavior has been studied.

Firstly, the deformation systems responsible for the polycrystalline plasticity at the grain level, in a hexagonal-close-packed, coarse-grained, and random-textured Zircaloy-4 alloy are considered. The evolution of internal strains was measured …

Actuatable Membranes Based On Polypyrrole-Coated Vertically Aligned Nanostructures, Benjamin L. Fletcher

Doctoral Dissertations

Nanoporous membranes are an enabling technology in a wide variety of applications because of their ability to efficiently and selectively separate molecules. A great deal of effort is concentrated on developing methods of externally controlling membrane selectivity and on integrating the membranes within multi-scale systems. In this dissertation, synthetic nanoporous membranes that fit the described needs are constructed from vertically aligned nanostructures. Vertically aligned carbon nanofibers and anisotropically etched silicon posts are aligned perpendicular to the substrate and act as obstacles to material flow parallel to the surface. The distances between the outer edges of the nanostructures define the pores …

Densification, Microstructure, And Mechanical Properties Of Zirconium Diboride Based Ultra-High Temperature Ceramics, Sumin Zhu

Doctoral Dissertations

"The first part of this dissertation was aimed at studying the densification of ZrB₂ ceramics by pressureless sintering techniques. Various processes have been applied to coat ZrB₂ powders with polymer precursors, which were used to produce C after charring. After sintering at 1900⁰C, relative density increased of ~70% for uncoated ZrB₂ to >99% for ZrB₂ coated with at least 1.0 wt% C. Thermodynamic analysis suggested that C reacted with and removed oxide impurities (ZrO₂ and B₂O₃) that were present on the ZrB₂ particle surfaces, which promoted densification by minimizing grain coarsening"--Abstract, page iv.

Geological Sequestration Of Carbon Dioxide By Hydrous Carbonate Formation In Steelmaking Slag, C. Hank Rawlins

Doctoral Dissertations

"The formation of carbonate solids from the alkaline earth oxide phases in steelmaking slag was investigated in dry and aqueous conditions as a vehicle for carbon dioxide sequestration. The goal of this research was to determine the process conditions that would increase the kinetics of the carbonate formation process under hydrous conditions, enabling direct removal of carbon dioxide from steelmaking offgas"--Abstract, page iv.

Modeling And Simulations Of Diphasic Composites For Development Of High Energy Density Dielectrics, Sandeep K. Patil

Doctoral Dissertations

"This research is focused on diphasic dielectric composites for high energy density storage applications in pulsed power and power distribution systems. Composite materials are particularly attractive because they can synergistically combine high permittivity with high breakdown strength of the individual phases. Energy density, which captures permittivity and dielectric breakdown strength as vital material parameters, dominates this research challenge. Avenues for increasing the effective permittivity and breakdown strength may be found by gaining fundamental understanding of the response of diphasic dielectrics to an applied electric field. Improved understanding of these characteristics will ultimately lead to dielectrics with increased energy storage densities. …

Thin Film Techniques For The Fabrication Of Nano-Scale High Energy Density Capacitors, James N. Reck

Doctoral Dissertations

Dielectric thin films of either TiO₂ or BaTiO₃ were sputtered in O₂/Ar plasmas on Si wafers to thicknesses ranging from approximately 25 to 200 nm with patterned Ni or Pt electrodes sputtered in Ar plasmas at thicknesses from about 20 to 250 nm to form nano-capacitors. Statistical design of experiments (DOE) was used to determine the effects of the deposition power, plasma composition, and deposition temperature on the measured electrical properties of the nano-capacitors. Additional tests to determine the effects of the dielectric and electrode thickness on the measured dielectric responses of the devices were also undertaken. Characterization was performed …